The Old Trafford door is always open for Cristiano Ronaldo to return to Manchester United, as Sir Alex Ferguson admits he would always welcome back the goal machine.

The most expensive footballer on the planet left the Reds in 2009 to join his boyhood team Real Madrid – and with an £80million price tag there was no question about letting him go.

His 165 goals in 162 games is bettered only by arch rival Lionel Messi, and he will be hoping to get one over on another old foe when the Spanish champions arrive in the Manchester to take on City this Wednesday.

"He’s an unbelievable player," Sir Alex told the club’s official website. "One of the best, no doubt about that.

“We have had a great pantheon of players but no question Ronaldo compares with any of them. In his last season, he made five chances every match – he's a brilliant player."

The colossal switch made no difference to Ronaldo’s attitude to succeed, his role in Jose Mourinho’s squad has flourished after being pushed into a striker’s role.

And despite Fergie enjoying his ‘greatest-ever’ attacking force with Wayne Rooney, Robin van Persie, Danny Welbeck and Javier Hernandez – he admits that he would enticed by a buy-back option.

When asked if he regrets not including a contract clause for his return to United, Sir Alex said: "How do you know I haven't? He always wanted to play for Real, it was a boyhood dream, but I’m not saying he’ll stay there forever. He might leave, you never know.

"I don’t think it’ll happen but I would like to think it would [be].

"We’d be talking incredible amounts of money now. What do you think his value would be? He’s never injured. When does he ever miss a game? Who could afford him? One of the Russian clubs could buy him but do you think he’d go to Russia?

“He complained about the weather here so imagine him in the middle of winter in Russia!”

And despite being one of the most famous sportsmen on the planet, the Scotsman still manages to maintain a relationship with his former winger – insisting that he still holds United close to his heart.

"I speak to Cristiano quite regularly,” he added. “He’s always available to talk to, and he’s always wishing us well. I think our result is the first one he looks for, no doubt about that."

For more on this story and many others, follow Mancunian Matters on Twitter and Facebook.